Many machines over many decades have included cooling fans for circulating air over a component, such as an engine, to be cooled during normal operation of the machine. The cooling fan and engine may be housed in a common engine compartment that includes openings to define an air flow passage through the compartment. Typically, these openings are covered with some screening material or other strategy for preventing debris from entering the engine compartment. In some machines, such as off road construction machines, landfill machines and the like, debris can collect on these screens during machine operation, resulting in a decrease in cooling air through the various coolers located in the engine compartment and a degradation of the cooling of the machine. Coolers include, but are not limited to hydraulic oil, transmission oil, intake air, etc. The cooling air may also circulate over the engine. Over the years, a variety of strategies have been developed to deal with this phenomenon. One known strategy is to periodically and briefly reverse the cooling air flow direction to dislodge any debris that may have collected on the screen surface. This may be accomplished either by reversing a cooling fan's rotation direction or even altering cooling fan blade orientations.
In one specific example taught in co-owned U.S. Pat. No. 7,240,486, an electro-hydraulic fan system includes a hydraulic motor for rotating the fan to circulate cooling air about an engine. In order to periodically dislodge debris that may have collected on a screen covering the air flow passage entry into the engine compartment, an electrical actuator is energized to move a valve to change the flow direction of hydraulic fluid being supplied to the fan motor. Although the fan motor is typically decelerated before the fluid flow reversal event takes place, the fan motor may continue to have rotational momentum in one direction even after the flow reversing direction valve actuator is energized. When this occurs, the residual angular momentum in the motor causes it to briefly act as a pump resulting in a pressure surge or spike at one port and a severe pressure drop or vacuum at its other port until the motor reaches zero speed before reversing direction. The '486 patent utilizes auxiliary check valves to open a fluid connection between the low pressure tank and the port undergoing a vacuum during the fan reversal process. As such, vacuum conditions can be somewhat alleviated as fluid from the tank can flow to the low pressure port. However, this does little to alleviate, and may actually exacerbate the pressure spike at the other port. Since the fan reversal motor, and the associated valves and pump can be expected to experience the consequences of a multitude of fan reversal events, problems and premature degradation in these components can be revealed due to the repeated cavitation/pressure spike events that typically accompany a fan reversal process.
The present disclosure is directed toward one or more of the problems set forth above and to reducing cavitation and/or pressure spikes in reversing hydraulic motor systems.